This is the Bonus Level where you get to fight Otto Giftmacher.

It's heavy fighting from the start and throughout this level.
The Gold Key is in the southern grey brick section. Push the wall in the second room by the brown plants, then the block with the drain making the Gold Key appear. Get into the central room and push the wooden blocks and by that blocking the retreat. Another secret door leads into the central red brick block which holds a 1-up, and an SS.
Now make it into the northern wood panel section. Handle the enemies in the first rooms, run past Giftmacher and into the next room, finish quickly with the officers and SS in this room and grab the chaingun. Move through the next door for the Silver Key. Now you must think fast! Either engage in a heavy closeup battle with a whole bunch of officers and SS or make it back past Giftmacher to the red brick area. Here you are now able to enter the rooms with the powerups, and more officers and SS.